Plant Legumes to Help Add Nitrogen to Your Garden Soil

Crop rotations in which you plant legumes will lead to better garden soil and bigger garden harvests.

| January/February 2015

  • Broad bean, also known as fava bean, is a hardy plant grown all over the world.
    Photo by Fotolia/Witold Krasowski
  • Green beans are an effective nitrogen-fixer in the garden; they're also nutritious and delicious.
    Photo by iStockphoto.com/Philary
  • Soybeans are typically grown in a corn-soybean rotation in commercial agriculture.
    Photo by iStockphoto.com/fotokostic
  • The pinto peanut is quick-growing and can cover up to 6 feet of ground per year, ideal for cover plants to grow under tree cover.
    Photo by iStockphoto.com/ntdanai
  • Young chickpea pods; chickpeas will grow to 8 to 20 inches in height.
    Photo by iStockphoto.com/bdspn
  • Soybean plants could all serve as cover crops and be implemented in a home gardener’s crop rotation plans.
    Photo by iStockphoto.com/DLeonis
  • Green beans on a trellis grow next to lettuce, tomato plants and herbs. Maintaining clearly defined beds or plots helps with rotation plans.
    Photo by iStockphoto.com/BasieB
  • Alfalfa is popular as cover crop and fodder for the animals.
    Photo by iStockphoto.com/Digi4
  • Red clover is a popular choice for green manure.
    Photo by iStockphoto.com/Whiteway
  • While cow vetch (Vicia cracca) can serve as a cover crop, and is even used as a forage crop for cattle, in some areas it can also be an invasive weed.
    Photo by iStockphoto.com/RuudMorijn

Crop rotation is a part of both traditional and organic agriculture. Its roots extend back to around 6,000 B.C., not too long after the dawn of agriculture in the Fertile Crescent. The basic idea is that the farmer grows something different on a given plot of land every year. Crop rotation has many benefits, including reducing the damage from pests and disease, and retaining the nutrient balance in soil. An increase in yield of 10 to 25 percent has been found in studies of farms that practice crop rotation, and — if you include legumes in your rotation — you can avoid depleting the soil of nitrogen by doing so. With a little planning and record keeping, you can bring the benefits of crop rotation to your home garden.

Benefits

Crop rotation results in increased yields, although scientists are not sure exactly why. There are several factors that may contribute.

First of all, if you grow the same crop on the same plot of land year after year, you are enriching the soil and environment with pathogens and pests of that crop. For example, if you grow squash and your plants attract squash vine borers, the borer will consume the vine and then lay eggs that will develop into grubs in the soil. The next season, the grubs will emerge. If squash is planted in the same patch, the insects do not have to search for food. The same basic idea applies to soil bacteria, fungi and viruses. Growing the same crop on the same plot leads to a cycle of disease.

Secondly, different crops have different nutrient needs. All plants fix carbon dioxide from the air to meet their needs for carbon. Plants also need nitrogen, phosphorus and potassium as major nutrients. (Nitrogen, phosphorus and potassium are the “N,” “P” and “K” listed on bags of fertilizer.)



In addition, plants require smaller amounts of micronutrients, and different crops have different requirements. For example, in many fruits — including tomatoes — calcium is required at levels almost as high as nitrogen, phosphorus and potassium. Planting the same crop over and over on the same plot of land runs the risk of depleting key micronutrients. In addition, plants differ in the amount of nitrogen, phosphorus and potassium they require, and macronutrient deficiencies can arise if a “heavy feeder” is planted in the same location year after year.

Including both deep-rooting and shallow-rooting plants in a rotation may also help improve the soil. Deep-rooting plants help break up the deeper, compacted layers and soil, while more shallow-rooted plants help aerate the soil near the surface.






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